Skis have various characteristics that affect the ease of skiing and the ability of the skier to achieve or fail to achieve a high level of proficiency. Thus, a good ski exhibits straight line stability at high speed, and also the ability to turn easily and to absorb bumps and ripples in the snow. In addition, it should show an ability to traverse across a steep slope without side slip.
Skis include a core, which may be made of various materials, such as wood, foam, honeycomb and various laminated materials. The core controls most of the resilience of the ski.
The bottom surface of the ski is made of a material which is slick, such as a suitable plastic that slides very well over the snow, and this is bonded to the bottom of the core.
The sides and the top may be of wood or of plastic or metal, but are usually of a type of material different from the bottom because slipping and sliding is not their function. The sides and top are also bonded to the core, and are preferably surrounded with a waterproof covering, which may also be decorative.
The bottom is usually provided with metal edges that function to cut into the ice or hard snow, so that the ski can bite and hold a turn without sliding sideways. These metal edges also help when traversing a slope and when the skier wishes to stop. The skier himself causes these metal edges to bite into the snow by angling his legs, and thus the skis, in the direction of the turn.
Good skis are usually narrower in the center than at the tips when viewed in plan, so that the sides to which the metal edges are affixed form a large arc. This arc helps to cause the ski to start turning when it is angled. The longer the portion of the metal edge that cuts into the snow, the more lateral force th ski can exert to enable ths skier to turn sharper and more quickly. Skis are limber and do not twist uniformly along their length when the skier angles them to turn.
Heretofore, such skis have been weak in torsion, so that the tips, the front, and the rear do not angle as much as the center, where the ski boot is attached as by bindings. This weakness in torsion forces the skier to accentuate the angularity of his legs, and the center of the ski does not achieve the same edge hold that would be obtained if the ski did not twist. such a twisted ski cannot exert the amount of force in a turn that an untwisted ski can, and is therefore a source of inefficiency.
Another important quality in skis is their compliance, their flexurability in the vertical direction. A highly compliant ski makes the ride smoother over the snow, enables the skier to maintain his balance more easily, and achieves a relatively even pressure distribution along the length of the ski, as applied to its bottom surface. Pressure along this bottom surface is a factor in making skis run fast. Areas of extreme pressure due to low compliance are certainly not desirable.
While a ski could be made to be very stiff in torsion by making it much thicker, it would then be much less compliant when moving over ripples and bumps and deep depressions in the snow. The overall result would then be very undesirable.
The compliance ability of a ski relates to its stiffness or flexibility in beam. It is desirable to have high compliance, and so it is desirable for a ski to be relatively weak in beam. On the other hand, twisting of the ski takes place because a typical ski is very weak in torsion. Both of the types of action may occur separately or simultaneously, depending on the terrain and on the action of the skier.
In the past, skis have been relatively weak in both beam and torsion. It is easy, as stated above, to make such a ski stiff in both beam and in torsion, but it has been nearly impossible to make the ski weak in beam, and yet stiff in torsion, and yet that is what is basically desired in a ski.
An object of the present invention is to accomplish stiffness in torsion while leaving the beam flexible, or relatively weak, so that the beam strength is relatively low, but the torsion stiffness is high.
Another object of the invention is to enable a designer to control, almost independently, each of the two factors, torsion and beam strength. Usually, this will be effected by starting with a ski design that is weak in beam and weak in torsion, and by applying the principles of the present invention to increase the torsional rigidity without substantially affecting the beam flexibility.